3-Iminonitriles are useful as intermediates for synthesizing photographic couplers or medicines. For example, 3-iminobutyronitrile is used in JP-A-59-171956 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") corresponding to U.S. Pat. No. 4,540,654 as a starting material for synthesizing 1H-pyrazolo[1,5-b]-1,2,4-triazoles useful as photographic magenta couplers.
As to synthesis of the 3-iminonitriles, descriptions are given in Journal of the American Chemical Society, 64, 150 (1942), French Patent No. 1,377,891, Canadian Journal of Chemistry, 43, 332 (1965), and Austrian Patent No. 375,067, disclosing, for example, the following synthesizing processes: ##STR3##
However, these processes result in the undesired production of poisonous sodium prussiate. In addition, in actual production, a step of removing the sodium prussiate is inevitably involved, and therefore the processes involve an additional problem in that production cost of the 3-iminonitriles becomes seriously high.
Further, The Journal of Organic Chemistry, Vol. 16, February, 1951, PP. 165-172 describes the following synthetic reaction: ##STR4## and, in specific embodiments thereof, compounds of formula (C) wherein R is methyl and phenyl are reported as being obtained in a yield of 20% and 78%, respectively, calculated based on the yield of .beta.-ketonitrile obtained by acid hydrolysis of the compound of formula (C). However, with the use of acetonitrile as a starting material of the above reaction, diisopropylaminomagnesium bromide (DIPAM) or sodium is used as a basic reagent. The use of sodium as a basic reagent provides the desired product in high yields, but results in generation of poisonous sodium prussiate as described above, and the use of DIPAM as a basic reagent provides the desired product in a yield of only about 5%.
The above J. Org. Chem reference describes that the use of propionitrile (CH.sub.3 CH.sub.2 CN) and DIPAM as a basic reagent at a molar ratio of 1:1 produces the desired compound (C) in 65% yield, whereas the use of acetonitrile and DIPAM at a molar ratio of 1:1 produces the desired compound (C) in only 5% yield. Also, this reference describes that, when propionitrile and NaH as a basic reagent are used at a molar ratio of 1:1, the desired compound (C) can be obtained in a yield of 20%, with a considerable amount of unidentified solid also being produced. Thus, the above J. Org. Chem. reference teaches that (1) the yield of the desired compound produced from acetonitrile is very low as compared with that produced from propionitrile when DIPAM is used as a basic reagent; and (2) the yield of the desired compound produced from propionitrile using NaH as a basic reagent is low as compared with that is obtained by using DIPAM. Contrary to the teachings of the J. Org. Chem. reference, the present invention makes it possible to produce the desired 3-iminonitrile, a dimer of acetonitrile, from acetonitrile using a small amount (from 0.01 to 0.5 molar equivalent) of a metal hydride such as NaH, without producing a trimer of acetonitrile.